I was bored at work so I started thinking of crossover ideas-
Jak and Daxter/Digimon- One of the cut features for Jak 1 was an animal that Jak could raise. So i thought about making Jak a Tamer in a sequel to the first game. Jak wanted to be an explorer so travelling between the digital world and his world feels like a good mechanic. Jak can already fight so you could have Data Squad like mechanics where Jak fights digimon with his partner or Frontiers were Jak transforms into one of the spirits.
Worlds ends with you/Yu yu hakusho- I thought of a Yu yu hakusho and Worlds end with you crossover where Koenma uses the Reaper's game as a recruiting test for Spirit detectives. So working for koenma is what Neku and the others did after the first game.